The fact that you have paid rent/mortgage is irrelevant.
Brother and Father own a house.
Brother and Father have a mortgage of €90k.
Say the house is worth €300k
1) They will sell the house to you for €300k
2) You will need a deposit of €30k and a mortgage of €270k
3) You pay them €300k and they repay the loan.
If they agree to sell the house to you for €90k it will be very messy.
1) Brother and father sell a house worth €300k for €90k to Brother 2 (it's simpler if it's to Brother 2 and not his wife)
2) Brother 2 will be getting a gift of €210k and this will be subject to Capital Acquisitions Tax.
3) The gift of €105k from the dad will be below the Threshold of gifs from father to son so no CAT payable
4) The gift of €105k from Brother to brother will be €70k in excess of the Class B threshold, so gift tax of about 33% or €22k will be payable.
5) Brother 2 will have to get a new mortgage of €90k from any lender - not necessarily the existing lender, but they might oblige.
Capital Gains Tax
As Brother and father are selling an investment, if it has increased in value, they will be subject to CGT on any gain in value since they bought it.
For example, if they bought it for €200k and it's now worth €300k, they will have to pay about €33k CGT.
Brendan